The activated carbon used for an electrode application in an electric double layer capacitor is produced by carbonizing, granulating and activating a material composed of carbon. The activated carbon has an extremely large number of pores. By forming an electrical double layer at the surface of these pores, electric power can be stored. However, an electric double layer capacitor has a smaller discharge capacity compared with a secondary battery. A further increase in the discharge capacity of electric double layer capacitors is therefore being needed. To increase the storage capacity density of an electric double layer capacitor using an activated carbon electrode, attempts have been made to increase the specific surface area of the activated carbon (i.e., a surface area per unit weight), but since the bulk density is reduced, it is difficult to increase the specific surface area over that of current products to increase the storage capacity density.
To solve the above problem, attempts have been made to utilize carbon nanofibers as the carbon material for the electrode material (see, for example, Patent Document 1). Carbon nanofibers form a network structure, and therefore the holes in the network act as pseudo pores and are effective for obtaining a large capacity, but these are bulky, and therefore, the electrode density is decreased and the capacity per volume is decreased. Further, carbon nanofibers are expensive, and therefore, an increase in cost of the electrodes is incurred. Further, attempts are made to use conductive polymer/porous carbon composites of activated carbon or another porous carbon material and a conductive polymer as an electrode to increase the storage capacity density of an electric double layer capacitor.
Patent Document 2 and Patent Document 3 propose preparing a conductive polymer/porous carbon composite by the electrolytic polymerization method and using it as the polarizable electrode of an electric double layer capacitor and use a polyaniline/porous carbon composite as the electrode. According to these Patent Documents, there are the advantages that the specific electrostatic capacity is larger than the case of use of a conventional polarizable electrode and the internal resistance also becomes smaller. However, the electrolytic polymerization method has the problem that the area of the resultant electrode is limited, and therefore, polymerization over a large area is difficult and not practical. Further, Patent Document 4 proposes to chemically polymerize aniline in an aqueous solution in the presence of a porous carbon material to obtain a polyaniline/porous carbon composite and to use this as a polarizable electrode, but there is the problem that the resultant polyaniline/porous carbon composite has to be rinsed, and therefore, the operation becomes complicated. Further, Patent Document 5 proposes to mix an aqueous polyaniline sulfonic acid solution and porous carbon material, then distill off the water in vacuo to obtain a polyaniline/porous carbon composite and to use this as a polarizable electrode, but a polyaniline sulfonic acids is water soluble, and therefore, in a water-based electrolytic solution, the electrode would easily leach the polyaniline sulfonic acid, while in an organic solvent-based electrolytic solution, the affinity of the electrode with the electrolytic solution would be low and, furthermore, the water used at the time of production could not be completely removed from the electrode, and therefore, an electric double layer capacitor using an organic solvent-based electrolytic solution would have the problem of an inferior long term stability. Further, a polyaniline sulfonic acids have a sulfonic acid group at the side chain thereof, and therefore, there is also the problem that the withstand voltage of the electrode will becomes lower depending on the selected electrolytic solution.
Patent Document 6 proposes a battery electrode composed of polypyrrole, polypyridine, or another π-conjugated polymer including a nitrogen atom and a carbon material or other conductive aids. However, the battery electrode described in Patent Document 6 is a secondary battery electrode, the weight ratio of the π-conjugated polymer and a conductive aid is 70:30 to 97:3, and the main substance is the π-conjugated polymer. Further, since the main component substance of the electrode agent is a π-conjugated polymer, when making the film thickness of the electrode 50 μm or more, there is also the problem that cracks etc. occur on the electrode surface and the electrode will not sufficiently function. Further, when using polypyridine or another π-conjugated polymer including a nitrogen atom together with a carbon material and the content of the carbon material is greater than that of the π-conjugated polymer, it will function in the same way as the polyaniline as a capacitor electrode material. By the combined use with a π-conjugated polymer, the electrostatic capacity was improved, compared with an electrode composed of only a carbon material, but it was not necessarily satisfactory.
Patent Document 7 discloses a method for producing a vinyl-based conductive polymer fiber by the electrospinning method, but has no description relating to an electrode material for a capacitor. Further, a polyparaphenylene vinylene or other vinyl-based conductive polymer has a lower conductivity than other π-conjugated polymers and is insufficient in performance as an electrode material for a capacitor. Patent Document 8 discloses a solar battery electrode formed from a mixture of a conductive polymer and a carbon nanotube by the electrospinning method, but no detailed and specific examples relating to a conductive polymer and a carbon nanotube are shown. Patent Document 9 also discloses a solar battery electrode obtained by the electrospinning method, but this is an electrode composed of only a carbon material.
Patent Document 1: Japanese Patent Publication (A) No. 2004-193443
Patent Document 2: Japanese Patent Publication (A) No. 7-201676
Patent Document 3: Japanese Patent Publication (A) No. 2002-25868
Patent Document 4: Japanese Patent Publication (A) No. 2002-25865
Patent Document 5: Japanese Patent Publication (A) No. 2003-17370
Patent Document 6: Japanese Patent Publication (A) No. 2000-82467
Patent Document 7: Japanese Patent Publication (A) No. 2005-330624
Patent Document 8: Japanese Patent Publication (A) No. 2006-216562
Patent Document 9: Japanese Patent Publication (A) No. 2006-331790